The present invention relates to write drivers for an inductive head in a magnetic data storage system and more particularly to a method of accurately controlling a write driver""s current response by means of a circuit connected to an H-bridge to control current undershoot.
Conventional storage systems include an inductive coil to write information onto a recording surface of the magnetic medium, such as a magnetic disk. The inductive coil writes information by creating a changing magnetic field near the magnetic medium. A write driver circuit is connected to the inductive coil at two terminals. During writing operations, the write driver circuit forces a relatively large current through the inductive is coil to create a magnetic field that polarizes adjacent bit positions on the recording surface. Digital information is stored by reversing the polarization of selected bit positions which is done by reversing the direction of the current flow in the inductive coil.
The typical write driver circuit includes an xe2x80x9cH-bridgexe2x80x9d for controlling the direction of current flow through the inductive coil. The H-bridge includes upper xe2x80x9cpull-upxe2x80x9d bipolar transistors and lower xe2x80x9cpull-downxe2x80x9d bipolar transistors. The upper bipolar transistors are connected between a first supply voltage and the inductive coil terminals. The lower bipolar transistors are connected between another set of inductive coil terminals and a second supply voltage through a write current sink. The write driver circuit controls the direction of flow through the inductive coil by driving selected transistors in the H-bridge between ON and OFF states, thereby applying a limited voltage swing across the inductive coil to reverse the coil""s current flow and to polarize the adjacent bit position on the magnetic medium.
The rate at which information can be stored on a recording surface through an inductive head is directly proportional to the rate at which the direction of current can be reversed in the inductive coil. The rise/fall time of the inductive coil is determined by:
di/dt=V/L
where di/dt is the rate of change of the current over time through the inductive coil, V is the available voltage across the inductive coil, and L is the inductive load. Therefore, the rate of current change through the coil is directly proportional to the available voltage across the inductive coil. The available voltage is determined by subtracting the voltage drops across the H-bridge pull-up transistors, the pull-down transistors, and the write current sink from the supply voltage.
In addition to the rate of current change through the coil, there are other coil current attributes that will affect how magnetic transitions are written to the medium. Some important coil current characteristics are shown in FIG. 7. In particular, the current""s rise time (rate of change), overshoot, undershoot, and settling time are of interest. The desired characteristics for the coil current are a fast rise time and settling time, a controllable amount of overshoot, and very little undershoot.
When the H-bridge switches the direction of current through the coil, a xe2x80x9cflybackxe2x80x9d voltage is produced on the current sourcing side of the H-bridge. A coil current undershoot results from this flyback voltage. The flyback voltage can be used to determine when the H-bridge current source should be compensated to reduce the coil current undershoot.
The write driver circuit of the present invention accurately controls the current through the coil that is used to write data to the magnetic medium.
The present invention provides an undershoot circuit to accurately adjust the amount and duration of coil current undershoot over a wide range of write current settings and undershoot current settings.
This invention compares the flyback voltage of the coil to determine when a compensation current should be applied to reduce the coil current undershoot. By using the flyback voltage, the compensation circuit can turn on precisely when the undershoot compensation is needed.
Furthermore, because the undershoot is controlled at the base of the bottom of the H-bridge circuit, the undershoot circuit of the present invention requires little power dissipation and adds little parasitic capacitance to the H-bridge.